Electro-pneumatic controllers (also called electro-pneumatic regulators) are typically used to precisely control the pressure of control fluid used to position a pneumatically-controlled device. For example, an electro-pneumatic controller can be used to control the pressure of control fluid into a pneumatic actuator of a process control valve or regulator to precisely position the process control valve or regulator. The electro-pneumatic controllers may include one or more passageways to receive the control fluid, and one or more solenoid valves may control the flow of the control fluid through the passageway of the electro-pneumatic controllers. The one or more solenoid valves may be in communication with a microprocessor and associated circuitry, and the microprocessor may be in communication with one or more sensors disposed outside of the electro-pneumatic controller (e.g., within an interior portion of the pneumatic actuator of the process control valve or regulator) to provide closed-loop control of the electro-pneumatic controller and/or the process control valve or regulator. The one or more passageways, one or more solenoid valves, and the microprocessor and associated circuitry may all be disposed within a housing that allows the electro-pneumatic controller to act as a modular device. Because inert gas is typically used as a control fluid, there is no danger of the microprocessor or associated circuitry igniting the control fluid.